(a) Field of the Invention
The present invention relates to a fuel cell system, and more particularly, to a stack for a fuel cell system.
(b) Description of the Related Art
As is well known, a fuel cell is an electricity generating system for directly converting chemical reaction energy into electric energy through an electrochemical reaction between hydrogen contained in hydrocarbon materials (e.g., methanol, ethanol, etc.) and oxygen or air containing oxygen.
The fuel cell that uses hydrogen generated by reforming methanol, ethanol, etc. as a fuel has a wide range of applications including as mobile power sources for vehicles, distributed power sources for homes or buildings, and small-sized power sources for electronic apparatuses.
The fuel cell described above has a structure in which a unit cell (or cell unit) includes a membrane-electrode assembly (MEA) for generating electricity by oxidation/reduction reaction between hydrogen and oxygen, and separators (or bipolar plates) that are respectively disposed on opposite surfaces of the MEA to supply hydrogen and oxygen to the MEA. A stack includes a plurality of unit cells.
The stack is composed of an additional pressing plate in contact with an outermost unit cell on either side of the stack, and may have a construction in which the unit cells are fastened by joining a restraint element of the pressing plate using a restraint rod and a nut.
Conventionally, when the pressing plate is joined to assemble the stack by using the restraint rod and the nut, there has been a problem in that a stress force is concentrated only at a portion of the pressing plate where the nut comes in contact with the pressing plate. This is because the pressing plate is deformed by a force generated when the nut is joined with a screw thread formed at the restraint rod, and the nut is then squeezed (or pressed) by the pressing plate, and thus the entire surface of the nut comes in contact with the pressing plate only in a particular surface portion of the pressing plate rather than coming in contact therewith in a uniform manner.
Accordingly, a restraint force of the restraint element of the pressing plate is weakened, and thus the stack becomes less resilient.
In particular, the aforementioned problems become more apparent when the thickness of the pressing plate of the stack is equal to or less than 2 mm.